A widely used and successful commercial process for synthesizing acetic acid involves the catalyzed carbonylation of methanol with carbon monoxide. The homogenous catalysts contain rhodium and/or iridium and a halogen promoter, typically methyl iodide. The reaction is conducted by continuously bubbling carbon monoxide through a liquid reaction medium containing the homogenous catalyst. In addition to the catalyst, the reaction medium also comprises methanol, methyl acetate, water, and methyl iodide. The carbonylation product is withdrawn from the reactor and separated in a flasher into a catalyst-containing solution, which is typically recycled to the reactor, and a vapor product stream. Further purification and separation of the vapor product stream yields the desired acetic acid. The homogenous catalyst, whether in the reactor, flasher, or being transferred between the reactor and flasher, should remain relatively stable to prevent loss of catalytic function through, for example, precipitation.
As developed, the rhodium-catalyzed carbonylation process employed a water concentration of over 14 wt. % based on the weight of the total reaction medium, as described in U.S. Pat. No. 3,769,329. Water enhances the reaction rate at the expense of purification capacity. Also, a substantial amount of energy is required to remove water. To reduce the need for water removal, “low water” processes were subsequently developed. Examples of these low water processes include those described in U.S. Pat. Nos. 5,001,259; 5,026,908; and 5,144,068, which are hereby incorporated by reference. These low water carbonylation processes maintain catalyst stability and increase productivity at high levels by maintaining an iodide salt in the reaction medium. Even under these conditions, however, catalyst instability and catalyst precipitation may be problematic. U.S. Pat. No. 4,994,608, the entirety of which is incorporated herein by reference, relates to catalyst stabilization via an increase in hydrogen partial pressure in the reaction medium. In addition, increases of carbon monoxide partial pressure may also help stabilize the catalyst in the reaction medium.
Carbon monoxide, as a raw material, may contain hydrogen, which is a building block to heavy impurities. These heavy impurities require further purification and separation reducing operating efficiencies. In a typical continuous carbonylation process, the partial pressures of hydrogen and carbon monoxide are controlled by venting of the vapor-filled portion of the carbonylation reactor. The venting prevents the buildup of gases, e.g., hydrogen, carbon dioxide and methane, within the reactor. The buildup of these gases may lead to undesirable side reactions that reduce production yields and introduce impurities into the product acetic acid. Unreacted carbon monoxide is a major component of the vented gases, commonly comprising about 50 to 80 mol. % of the total vented gases. Thus, the venting of these gases results in a substantial loss of carbon monoxide reactant, which decreases overall carbon monoxide efficiency.
Readily condensable gases from the reactor are typically condensed and the resultant liquids, such as iodides and esters, are recovered and returned to the process. Non-condensable gases, such as carbon monoxide, may be purged from the system or recovered. U.S. Pat. No. 4,255,591, for example, describes recovering carbon monoxide by passing the reactor vent gas through a hollow fiber semi-permeable membrane to produce a non-permeated portion enriched with carbon monoxide. Similarly, U.S. Pat. No. 5,334,755 describes separating a gas portion of carbon monoxide and returning it to the reactor. U.S. Pat. No. 5,683,492 describes recovering carbon monoxide from a purge gas stream that is passed through a pressure swing adsorption process. The recovered gas fraction, enriched with carbon monoxide, is returned to the reactor. However, returning carbon monoxide to the reactor requires compressing the recovered gas fraction at high energy expense to a pressure that is equal to or greater than the pressure of the fresh carbon monoxide that is fed to the reactor.
Other efforts have sought to regulate the amount of carbon monoxide fed to the reactor. U.S. Pat. No. 7,476,761, for example, describes a control process for monitoring carbon monoxide concentrations to maintain catalyst stability. U.S. Pat. No. 6,255,527 describes a method of controlling the flow of carbon monoxide to prevent the flow of carbon monoxide from exceeding a calculated point. Even though the amount of carbon monoxide fed to the reactor is regulated in these cases, the excess carbon monoxide is either purged or fed back to the reactor.
In addition to maintaining catalyst stability in the reactor, it is also desirable to maintain catalyst stability downstream of the reactor, e.g., in the flasher, separation zone, and in the connecting pipes. Fresh carbon monoxide may be added downstream of the reactor, but this carbon monoxide generally carries over with the vapor stream from the flasher and is purged. U.S. Pat. No. 5,237,097 discloses an organic compound that is reacted with carbon monoxide in the presence of a Group VIII metal-containing catalyst. The liquid carbonylation product solution of this reaction is conveyed to a separation zone maintained at a lower total pressure than is the pressure in the reaction zone. Simultaneously with the conveyance of the liquid product solution to the separation zone is the introduction therein of a carbon monoxide-containing gaseous stream, the carbon monoxide therein contributing a partial pressure of up to 30 psia of the total pressure in the separation zone. No provisions are provided in U.S. Pat. No. 5,237,097 concerning the carbon monoxide introduced to the separation zone and the carbon monoxide would be subsequently purged when purifying the flashed portion of the liquid product solution. US Pub. No. 2008/0293996 describes using a portion of the non-condensable gases that contain carbon monoxide from the reactor to stabilize the catalyst in the bottom of the flasher. However, the carbon monoxide that is fed downstream of the reactor is purged after scrubbing with methanol.
The need remains for improved carbonylation processes with increased catalyst stability while maximizing reaction efficiencies.